An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Full color OLED devices are also known in the art. Typical full color OLED devices are constructed of three different color pixels that are red, green, and blue in color. Such an arrangement is known as an RGB design. An example of an RGB design is disclosed in U.S. Pat. No. 6,281,634. Full color organic electroluminescent (EL) devices have also recently been described that are constructed of four different color pixels that are red, green, blue, and white. Such an arrangement is known as an RGBW design. An example of an RGBW device is disclosed in commonly assigned U.S. Patent Application Publication 2002/0186214 A1. In an RGBW device, high efficiency white-emitting pixels are used to display a portion of the digital image content. This results in improved power consumption relative to an RGB device constructed of similar OLED materials.
A white-emitting EL layer can be used to form a multicolor device. Each pixel is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The organic EL layer is common to all pixels and the final color as perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340. Other examples of white light-emitting OLED devices are disclosed in U.S. Pat. No. 5,683,823, JP 07-142169, and U.S. Pat. No. 5,405,709.
Kido et al., in Science, Vol. 267, p. 1332 (1995) and in Appl. Phys. Lett., Vol. 64, p. 815 (1994), report a white light-producing OLED device. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to produce white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent material in a visible light range from bluish green to red. Recently, Deshpande et al., in Appl. Phys. Lett., Vol. 75, p. 888 (1999), published a white OLED device using red, blue, and green luminescent layers separated by a hole-blocking layer.
A problem in the application of white OLED devices, when used with color filters, is that the intensity of one or more of the red, green, and blue components of the emission spectrum is frequently lower than desired. Therefore, passing the white light from the OLED through the red, green, and blue color filters provides light with a lower efficiency than desired. Consequently, the power that is required to produce a white color in the display by mixing red, green, and blue light can also be higher than desired.
One way of improving the efficiency of an OLED device is the use of a microcavity structure. A reflector and a semitransparent reflector function, with the layers between them, to form a microcavity. The layers between the reflectors can be adjusted in thickness and refractive index so that the resulting optical microcavity resonates at a desired wavelength. Examples of microcavity structures are shown in U.S. Pat. Nos. 5,405,710; 5,554,911; 6,406,801; 5,780,174; and JP 11288786.
Microcavity devices, however, have a known problem in that color distortion can occur when viewed at varying angles from the normal viewing angle. This effect is described in U.S. Pat. No. 5,780,174. Microcavity devices are characteristically directional; the emission intensity also varies with viewing angle. See, for example, N. Takada, T. Tsutsui, and S. Saito Appl. Phys. Lett. Vol. 63, p. 2032 (1993) “Control of emission characteristics in organic thin-film electroluminescent diodes using an optical-microcavity structure”. Therefore, an OLED device using microcavity structures having reduced dependence of perceived color on the angle of view is desired.